A typical example of the ceramic substrate is illustrated in FIG. 1 and comprises a ceramic plate 1 of aluminum oxide sandwiched between two copper plates 2 and 3, and a heat sink 4 of copper. The copper plates 2 and 3 are bonded to both surfaces of the ceramic plate 1 through a liquid phase bonding phenomenon. Namely, the bonding surfaces of the copper plates 2 and 3 are firstly oxidized, and the ceramic plate 1 is laminated with the copper plates 2 and 3 on both surfaces thereof. While the ceramic plate laminated with the copper plates 2 and 3 are heated at 1065 to 1085 degrees in centigrade, the copper and the copper oxide are melted, and the copper plates 2 and 3 are bonded to the ceramic plate 1 through a copper-and-copper oxide eutectic phenomenon. One of the copper plates 2 and 3 provides a conduction path between circuit components, and the other is soldered to the heat sink member 4 at lower than 450 degrees in centigrade.
A problem inherent in the prior art ceramic substrate is a small resistance against stress of thermal fatigue. When the electric or electronic circuit is activated for a task, a large amount of heat is produced in the circuit components, and the ceramic substrate conducts the heat for radiating. Since the copper plates 2 and 3 are different in thermal expansion coefficient from the ceramic plate 1, the ceramic plate is subjected to thermal stress. Upon being inactivated, no heat takes place, and the ceramic plate is released from the thermal stress. Thus, heat cycle takes place in the electric or electronic circuit. Since the aluminum oxide plate 1 and the copper plates 2 and 3 are rigidly coupled through the copper-and-copper oxide eutectic phenomenon, the ceramic substrate is repeatedly subjected to the thermal stress. The thermal stress is causative of cracks in the ceramic plate 1, and the ceramic substrate is broken in the worst case.